In an effort to improve grip performance, fuel economy, and abrasion resistance, silica having a large BET specific surface area (fine particle silica) is added to rubber compositions for tires. Fine particle silica, however, has a strong tendency to agglomerate, and thus is difficult to uniformly disperse in rubber compositions. This poor dispersibility of fine particle silica in a rubber composition causes reduction in elongation at break of the rubber composition containing fine particle silica.
Though manufacturers of silica are trying to improve dispersibility of fine particle silica by controlling the surface activity and particle size distribution of fine particle silica, fine particle silica with a satisfactory dispersibility still remains to be developed. Moreover, in some cases, the use of a modified polymer whose end is modified for silica can end up preventing dispersion of silica because the polymer may bond to silica before silica is sufficiently dispersed.
Uniform dispersion of fine particle silica in rubber is thus desired to provide a compounded rubber excellent in elongation at break.
Meanwhile, rubber compositions for tires generally contain sulfur. However, sulfur dissolved in a polymer has an S8 structure, a melting point of 113° C., and a polarity (solubility parameter (SP): 10) close to that of carbon disulfide. Such sulfur is thus generally difficult to uniformly disperse in diene rubbers with low polarity (SP=8-9), such as natural rubber, butadiene rubber and styrene butadiene rubber, which are widely used for rubber compositions for tires.
To use less amount of S8 sulfur, techniques using a sulfur-containing hybrid cross-linking agent (1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane), an alkylphenol-sulfur chloride condensate, or the like have been proposed. However, sulfur-containing hybrid cross-linking agents are generally expensive. Alkylphenol-sulfur chloride condensates are also poor in dispersibility, thus leading to deterioration in elongation at break or abrasion resistance.
Uniform dispersion of sulfur in rubber is thus desired to provide a compounded rubber excellent in elongation at break.
Meanwhile, tire components are required to have other properties, including handling stability and fuel economy. For example, the use of a tin-modified butadiene rubber, which strongly bonds to filler, has been proposed to improve fuel economy while maintaining handling stability.
Nonetheless, the effects of the above techniques are not sufficient to improve elongation at break. Further improved techniques are needed for providing handling stability, fuel economy, wet grip performance, elongation at break, and abrasion resistance together while maintaining the balance between them. Patent Literature 1, for example, discloses the use of a specific styrene butadiene rubber and a coumarone-indene resin to improve grip performance and the like, but does not discuss improving handling stability, fuel economy, or elongation at break.